Cells respond to stress by increasing synthesis of a number of heat shock proteins or HSPs, also known as molecular chaperones. The heat shock proteins assist general protein folding and prevent non-functional side reactions such as non-specific aggregation of misfolded or unfolded proteins. Heat shock protein 90 (HSP90) in particular has been identified as an important mediator of cancer cell survival.
Most HSPs are ubiquitously expressed under normal conditions, their basal levels facilitating normal protein folding and guarding the proteosome from the dangers of misfolding or aggregation, [Wegele et al, Rev. Physiol. Biochem. Pharmacol., 2004, 151, 1-44]. Under non-stressed conditions HSP90 comprises as much as 1-2% of total cellular protein content, whereas in tumour cells it is expressed at levels 2- to 10-fold higher in comparison to normal cells. These chaperone proteins are required for essential housekeeping functions, such as de novo protein folding during nascent polypeptide-chain synthesis, translocation of proteins across membranes and normal protein turnover. HSPs also participate in higher-order functions such as post-translational regulation of signalling molecules [Csermelt et al, Pharmacol. Ther, 1998, 79, 129-168], the assembly and disassembly of transcriptional complexes [Science 2002, 296, 2232-2235] and the processing of immunogenic peptides by the immune system.
HSPs function as components of multi-protein complexes which contain other chaperones, co-chaperones, modulators of ATPase activity and various accessory proteins. The components of the HSP90 chaperone complex include HSP70, HSP40, HIP, HOP, CDC37/p50, AHA1, p23 and immunophilin. Chaperones typically interact with client proteins in a cyclical, iterative fashion which is driven by ATP hydrolysis, [Smith et al, Mol. Cell Biol. 1995, 15, 6804-6812]. Targeting the nucleotide-binding pockets of HSP90 with small molecules may therefore provide a method of modulating the activity of the chaperone complex. HSP90 is unique amongst the chaperones as it is not required for biogenesis of most polypeptides. Many of its client proteins are conformationally labile signal transducers which are critical to cell growth and survival, [Pratt et al, Proc. Soc. Exp. Biol. Med., 1998, 217, 420-431]. Post-translational interactions with its clients allows HSP90 to couple stress response to changes in signal transduction pathways and transcriptional responses, [Morimoto et al, Cell, 2002, 110, 281-284]. Studies in Drosophila melanogaster have demonstrated that compromising the function of HSP90 can induce epigenetic alteration in gene expression as well as heritable alterations in chromatin state [Solars et al, Nature Genet. 2003, 33, 70-74 and Sangster et al, Cell Cycle, 2003, 2, 166-168].
A common feature of both solid and haematological malignancies is increased expression of one or more HSPs. Overexpression of HSP90 in breast cancer correlates with poor prognosis [Jameel et al, Int. J. Cancer 1992, 50, 409-415]. Increased chaperone expression contributes to oncogenesis at several levels. Increased abundance of HSPs in advanced cancers reflects an appropriate cytoprotective stress response to hypoxic and acidotic microenvironment of the tumour. At the molecular level increased chaperone activities appear to allow tumour cells to tolerate the deregulation in intracellular signalling associated with neoplastic transformation and thereby provide a mechanism for tumour cells to avoid apoptosis [Mosser et al, Oncogene 2004, 23, 2907]. The modulation of tumour cell apoptosis by HSP90 and its co-chaperones is mediated through effects on AKT, [Basso et al, J. Biol. Chem., 2003, 277, 39858-39866], tumour necrosis factor (TNF) receptors [Vanden Bergh et al, J. Biol. Chem. 2003, 278, 5622-5629] and NF-κB function [Chen et al, Mol. Cell, 2002, 9, 401-410].
Many kinases are client proteins of HSP90 including several which play a significant role in the progression of malignant phenotype such as HER2, AKT and RAF-1, [Neckers et al, Trends Mol. Med. 2002, 8, S55-S61]. Ligand-dependent transcription factors (e.g. steroid receptors), transcription factors (e.g. HIF-1α) containing PAS domains and mutated or chimeric signalling proteins (mutated p53, NPM-ALK kinase, p210Bcr-Abl) are also HSP90 clients. In addition some client proteins are involved other fundamental processes of tumorigenesis, namely apoptosis evasion (e.g. Apaf-1, RIP), immortality (e.g. hTert), angiogenesis (e.g. VEGFR, Flt-3, FAK, HIF-1) and metastasis (c-Met).
HSP90 resides predominantly in the cytoplasm, where it exists at a homodimer. Each monomer is comprised of three main domains. The N-terminal domain contains an unusual adenine nucleotide-binding pocket defined by a Bergerat fold, [Dutta et al, Trends Biochem. Sci., 2000, 25, 24-28]. Structural alterations driven by the hydrolysis of ATP in this fold appear to have an essential role in the chaperoning activity of the HSP90 dimer. In eukaryotes a highly charged linker sequence connects the N-terminal domain to the ‘middle region’ of HSP90. The structure of this middle region indicates that is has an important role in modulating ATP hydrolysis by interacting with the γ-phosphate of ATP molecules bound to the protein, [Meyer et al, Mol. Cell 2003, 11, 647-658]. The N-terminal ATP-binding site is also the site of interaction of the structurally unrelated natural products geldanamycin and radicicol. These compounds prevent the chaperone from cycling between its ADP- and ATP-bound conformations. Drug binding at the N-terminus of HSP90 seems to recruit E3 ubiquitin ligases such as CHIP (carboxy terminus of HSP70-interacting protein) to the many client proteins that are normally expressed by HSP90 protein complexes [Xu et al, Proc. Natl. Acad. Sci., 2002, 99, 12847-12852]. This recruitment leads to proteosomal degradation of the clients and depletion of their cellular levels.
In normal and many cancer cell lines, HSP90 inhibitors induce a predominant G1 cell-cycle arrest in a p53-independent manner, [McIlwrath et al, Cancer Chemother. Pharmacol. 1996, 37, 423-428]. Disruption of anti-apoptotic signalling in tumour cells occurs following exposure to HSP90 inhibitors and can enhance the pro-apoptotic effects of cytotoxic agents [Basso et al, Oncogene 2002, 21, 1159-1162].
We have now discovered a group of compounds which are potent and selective inhibitors of HSP90 and the isoforms and splice variants thereof. The compounds are thus of use in medicine, for example in the treatment of a variety of proliferative disease states, where inappropriate action of HSP90 may be involved such as cancer, inflammatory and immune disorders such as rheumatoid arthritis, psoriasis, Crohn's disease, ulcerative colitis, systemic lupus erythematosis, and disorders related to angiogenesis age related macular degeneration, diabetic retinopathy and endometriosis. The compounds may also be of use in the protection of normal cells against the action of cytotoxic agents. The compounds of the invention are related to the HSP90 inhibitors encompassed by the disclosure in International Patent Application Nos. WO 2003037860 and WO2002036075 as well as the publication J. Med, Chem 2006, 49, 817-829 but differ therefrom in that the present compounds have the amino acid ester motif referred to above.